Obesity is a major health problem, resulting in a substantial increase in diabetes, metabolic syndrome, and heart disease and severe strains on healthcare systems. Reducing obesity demands an in-depth understanding of its causes at the cellular, molecular and organismal level. In an exciting new development, rare human variants in the scaffold protein SH2B1 have been identified that associate with profound childhood obesity, insulin insensitivity, and in some individuals, maladaptive behavior and speech and language delay. Some of these mutations are unique to specific SH2B1 isoforms. In vitro, isoforms of SH2B1 have distinctly different subcellular localization and ability to enhance neurite outgrowth and gene expression. There is a fundamental gap in our understanding of how SH2B1 regulates the neuronal circuitry that regulates, body weight, insulin sensitivity, behavior and learning. Our long-term goal is to identify pathways regulated by the SH2B1 isoforms that are critical for the establishment and/or maintenance of neural circuits important for normal feeding behavior and energy balance. Novel mouse models, primary cultured neurons and well characterized cultured cells will be used to test the central hypothesis that each SH2B1 isoform makes a unique and crucial contribution to the establishment and maintenance of neural circuits important for normal feeding behavior and energy balance. This hypothesis will be tested by pursuing 3 specific aims: 1) Use novel in vivo mouse models to determine how the unique C-terminal tails of SH2B1 isoforms impact the function of LepRb-expressing neurons in the hypothalamus; 2) Determine at the cellular and molecular level how the C-terminal tails of SH2B1 isoforms regulate the function of SH2B1 in neurons with a particular focus on brain-specific SH2B1?; and 3) Define the role for SH2B1 isoforms in the regulation of body weight and insulin sensitivity. This research is innovative because: 1) SH2B1 was only recently implicated as a human obesity gene; 2) newly identified SH2B1 mutations in a unique cohort of obese individuals provide useful tools for studying causes of obesity; 3) the concept of coordinating an integrated response to neurotrophic factors by movement of scaffold proteins between the plasma membrane, cytoplasm, nucleus and nucleoli is novel; 4) present knowledge and tools now make it possible to study the mechanistic basis for the neuronal function of the various SH2B1 isoforms, their ability to regulate energy homeostasis in vivo, and their regulation of the transcriptome and neuronal circuitry of LepRb neurons; and 5) many of the proposed techniques and mouse models are cutting edge. These include CRISPR/Cas9 methodology to edit Sh2b1; novel mouse models to study the effect of SH2B1 on neuronal projections and gene expression of LepRb neurons relevant to feeding behavior and energy expenditure; and novel mouse models lacking specific isoforms of SH2B1 to study the impact of the specific SH2B1 isoforms in intact mice and isolated neurons.